It's not yet clear whether Jackson went into sudden cardiac arrest in his Los Angeles home, but that assumption has been made by many experts "on the basis of the report that his heart stopped, and he received resuscitation attempts," said Dr. Stephen Nicholls, a cardiologist at the Cleveland Clinic.

"The ultimate question is whether death was due to a problem with the heart or another problem," Nicholls said. An autopsy was performed Friday, but additional tests are to be performed and the results aren't expected for six to eight weeks, the Los Angeles County coroner's office said.

A heart attack happens when a coronary artery is blocked and some heart muscle dies. In sudden cardiac arrest, the heart simply stops beating, and the ventricles, the two blood-pumping chambers at the bottom of the heart, go into fibrillation, a useless fluttering.

When that happens, survival time is measured in minutes. The usual estimate is that the chance of survival goes down 10 percent for every minute that the heart stops beating. That means that Jackson probably could not have been saved, even though he was staying in a home that is only a six-minute drive from the UCLA Medical Center, where paramedics brought him for treatment.

The underlying causes of heart attacks and sudden cardiac arrests are often the same, said Dr. Kirk Garratt, director of the Heart and Vascular Institute of Lenox Hill Hospital in New York City.

"Most cardiac arrest is related to ischemic heart disease," Garratt said. Ischemia is blockage of an artery. "Most of the time, ventricular fibrillation takes place when an artery is blocked off. When that happens, if you don't get the rhythm corrected right away, you lose cardiac function and everything stops quite quickly."

Between 200,000 and 300,000 Americans die each year of sudden cardiac arrest, added Dr. Bruce Lindsay, a past president of the Heart Rhythm Society and director of cardiac electrophysiology at the Cleveland Clinic. "It is actually the most common cause of death in the United States," he noted.

The best emergency treatment is a shock from a defibrillator to restore heart rhythm. Defibrillators now are available in many public places, and tens of thousands of Americans have tiny defibrillators implanted in their chests each year.

Cardiopulmonary resuscitation (CPR), where regular thrusts are applied to the chest to keep blood flowing, can be a lifesaving technique "if it is done properly," Lindsay said. But the overall survival rate for people with sudden cardiac arrest is still only about 5 percent, he added.

There is a complex relationship between heart attack and sudden cardiac arrest, Lindsay said. "The majority of the time, sudden cardiac arrest is not associated with a heart attack," he said. "It is usually related to scarring or damage to the heart from some past event."

That past event might be a heart attack that caused scarring of the heart, Lindsay said. "Or it might be due to weakness of the heart muscle unrelated to heart disease, a virus or some other cause," he said.

Certain drugs can also cause cardiac arrest, Garratt added. On Friday, media reports said that doctors doing the autopsy on Jackson would be looking for possible effects of drugs. One report said that Jackson had an injection of morphine before the crisis. Another report mentioned possible use of the prescription narcotic Demerol.

"Usually, prescribed heart medications can stimulate abnormal heart rhythm if taken in large overdoses," Garratt explained. "So can medications that sedate somebody and make them not breathe properly, leading to low blood oxygen levels. Overdoses of sedative drugs like tranquilizers can have that effect."